Why do we get hot when lying in the sun

People are invisible to radio waves. In other words, there should be no effect when standing between a radio-wave transmitter and its receiver. Conversely, humans are not invisible to light in the wavelength range 100-1000 nm - we say they are opaque, which is most easily proved by asking a person to stand in the path of a light source such as a torch, and seeing the shadow cast. 

The crackle we sometimes hear when walking round a radio set is because of metals and other conductors in our clothing human flesh is invisible to radio waves. 

The science of the way light interacts with a species, such as a chro-mophore in red wine or a body in the sun, is called photochemistry. 

This principle is so simple that it has been given the title the first law of photochemistry, and was first expressed by Grotthus and Draper in the early 19th century. They stated it as the (hopefully) obvious truth Only light that is absorbed can have any photochemical effect . 

The primary interaction occurring during a photochemical process is between light and an analyte, such as a molecule, ion, atom, etc. The reaction is often written in the generalized form 

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